1. Field of the Invention
The present invention concerns an x-ray anode of the type having an emission layer and a carrier with carrier material to support the emission layer.
2. Description of the Prior Art
X-ray tubes include an x-ray anode and a cathode that are arranged in a vacuum enclosure. Electrons are thermally liberated from the cathode and accelerated by high voltage toward the anode where they are decelerated in an emission layer and generate x-rays. A large portion of the kinetic energy of the electrons is converted into heat that severely heats the x-ray anode during its operation. The power capacity of x-ray tubes is limited by this thermal loading of the x-ray anode. Various designs are known to increase the thermal capacity. For x-ray anodes executed as fixed (stationary) anodes, it is known to conduct heat from the x-ray anode via intermediate structures into a heat storage (heat accumulator or heat reservoir) made, for example, from graphite. For x-ray anodes executed as rotary anodes, the electron beam is directed onto a point on the surface of the plate-shaped x-ray anode at a distance R from the center point. By a fast rotation of the x-ray anode in operation, the heat is distributed along the focal ring described by the point and can additionally distribute during a rotation of the x-ray anode before the point is struck again by the electron beam. Cooling of the rotary anode with coolant is additionally known. A significantly higher capacity can be achieved than with fixed anodes. For rotary piston radiators it is known to rotate the entire x-ray tube in a bath of coolant and to thereby dissipate the heat from the x-ray anode.
It is common to all forms of x-ray anodes that the heat must be dissipated from the emission layer and conducted into a heat storage or a coolant. A carrier for supporting of the emission layer that is executed as an intermediate layer or directly as a heat storage, serves for this purpose. The emission layer is directly or indirectly applied.
From DE 10 2004 003 370 A1 it is known to produce this carrier from a combination made of a copper alloy for heat dissipation and a molybdenum alloy to impart the necessary stability. A very good heat dissipation can be achieved for highly heat-conductive graphite, but the problem exists that the coefficient of heat expansion of the graphite is not adapted to that of the emission layer. This leads to the situation that given a high loading of the x-ray anode small tears (fissures) arise due to the different expansion of the emission layer and the heat conductor. Such tears lead to a destruction of the x-ray anode.
To solve this problem it is known from DE 10 2005 015 920 A1 to insert a carrier made from one or more intermediate layers of carbon fiber material between the heat conductor (made from a carbon substance) and the emission layer, the carrier being backed with high melting point (refractory) metals. By varying the quantity of carbon fibers to metal, the coefficient of heat expansion can be adjusted in a certain range and thus a more densely stepped gradient of the coefficient of heat expansion can be achieved over a number of intermediate layers of the carrier. In this very stable solution, however, the heat conductivity of the carrier is unsatisfactory in a high capacity range of the x-ray anode.